Encryption and Digital Signatures

Encryption, in one form or another, has been around since almost the dawn of civilization. Computers and networks have made advanced forms of encryption possible. The kinds of encryption historically used (for example, secret decoder rings or German Enigma machines) are trivial to break on a computer, taking only seconds. Almost all versions of UNIX come with a program called crypt, which is a software implementation of the German Enigma machine used in World War II. A program exists on the Internet that breaks this code in seconds. Therefore, when dealing with encryption, always insist on using high-quality algorithms; weak ones can be broken easily.

Digital signatures are a way of securely signing documents. This strategy is useful for two reasons. The first is like a regular signature-to indicate that you agree with the terms of the document. The second is to detect whether someone has changed the document after you signed it (something that was unlikely with traditional paper documents). Digital signatures are implemented by making a digest of the message, using an algorithm such as MD5. This digest is encrypted by the user’s key. The recipient can decrypt the encrypted digest and compare it to the document. If the document doesn’t match, it has been tampered with or has a forged signature.

Legal Issues

When writing Internet programs that involve encryption, it’s very important to be aware of the legal issues involved. If you don’t, you could end up in prison on felony charges.

First, some countries ban outright any use of encryption. France and Iran are two notable examples. Therefore, you shouldn’t write any encryption code, import any encryption programs, or even use encryption in these two countries. Other countries might have such laws on the books. I recommend checking with an attorney unless you are absolutely sure of the legal status in your country.

The United States is another country with strange laws about encryption. No problem exists with using any form of encryption you want-within the country. The trouble starts when you want to write a piece of encryption software in the United States and then export it to another country (except possibly Canada) in electronic form. The U.S. government has labeled certain types of encryption software as munitions-just as if these programs were missiles or something. The law is known as ITAR (International Traffic in Arms Regulations).

It’s possible to write certain types of weaker encryption programs for export. You then have to request an exemption from the export law from the U.S. government. The level of encryption allowed is strong enough to be time-consuming for a cracker to break the code, but it can be done in a few months on some machines. For public key algorithms, covered in the section “Public Key Encryption,” the Software Publishers Association (SPA) and the government agreed to give quicker approval to algorithms using 40-bit keys or less. A message encrypted with a 40-bit key takes only about 200 MIPS/year of CPU time (that is, a 200 MIPS computer would take one year to crack it). Therefore, any highly secure, non-time-sensitive communication is potentially at risk. Keep this fact in mind.

A few ways exist to get around this law. The best is to just write your software outside the U.S. Some people ask the question, “What if my machine is out of the country, but I’m in the U.S.?” or vice versa. My answer is that there’s no legal precedent on this issue, so don’t take chances. The other alternative is that apparently you can print out the source code (or binary) to the program and carry the printouts out of the country without violating the law. Then, in the foreign country, you can scan them back in. Again, check with your lawyer before attempting this. Another method is for you to write a U.S. version of the program that you tightly control, and then have an associate write an international version outside the U.S. This has been done with PGP (Pretty Good Privacy), which is discussed later in this chapter.

To make matters even worse, the law even precludes giving the program to people in the U.S. who are not U.S. or Canadian citizens or permanent residents. I know this part of the law is widely violated, but my recommendation, as always, is to play it safe.

Another legal issue is that the major public key encryption algorithms are all patented. The patent owners are pretty aggressive about protecting their patents; however, many of the key patents expire in the next couple of years. The release of these patents will break the stranglehold the patent holders have had on public key encryption. I predict that after the patents expire we’ll see many more public key encryption implementations than we have today.

Private Key Encryption

Private key encryption is the oldest form of encryption in existence. In private key encryption, there is one key. You encrypt the message, using this key. The person decrypting the message must have the same key. It works just like the secret decoder rings you might have played with when you were a child.

The strength of private key encryption algorithms varies greatly. Some are trivial to crack; others are computationally impossible to crack, given today’s computers. Some vendors, notably WordPerfect, have included some private key encryption code in their products. It turns out that many of these are almost completely useless. You can download programs from the Internet to crack the keys, run them, and have the original file back in under 15 minutes (the download is the time-consuming part, too!).

The most famous private key encryption algorithm is DES (Data Encryption Standard). It’s very popular, and you can find implementations for almost any platform available today. However, DES is aging (originally published in 1975) and computers are getting faster. Attacks on DES are becoming increasingly possible. A modified version of DES known as Triple-DES is available and is harder to crack.

Public Key Encryption

The big problem with private key encryption is making sure that the people on both ends of the communication have the keys-without anyone else having them. The only ways to exchange keys in a secure fashion are to already have a secure communication channel available, which means that both parties already have encryption available; or both people must be in the same place without anyone else around, so that they can be sure they’re not being bugged. This is very cumbersome for most people.

Public key encryption eliminates the need for a secure key-exchange mechanism. Each person has a private key, which he uses to decrypt or digitally sign messages, and a public key, which others use to send messages to him or to verify his digital signature. Each person keeps his private key private to himself. The public key is public information and can be known by anyone.

Key Length

The length of the key is a critical issue in the security of a public key encryption algorithm. The longer the key, the safer your encrypted messages but the longer it takes to encrypt or decrypt them. My advice is to go ahead and use a long key (1024 bits or more). Most people today have enough CPU horsepower to encrypt messages to you quickly, even with a long key. This isn’t true when breaking messages without knowing the key, however. The longest message known to be broken had a 429-bit key. It required an international effort, involving 600 sites. The good news is that the amount of time needed with current cracking algorithms doubles with every additional 10 bits of key length. However, computer performance and algorithms to break public key encryption algorithms are improving, making cracking easier. Therefore, a long key is essential in protecting your messages for a long period of time.

Key Exchange

With public key encryption, the main implementation difficulty changes from key exchange to identity verification. If someone e-mails you her public key across the Internet, how do you know it’s really from her? The answer is that you don’t. The secure way is to get together with the person, in person, and exchange public keys. Well, if you do this, you almost might as well have used private key encryption and exchanged keys. Also, this model doesn’t scale well. Obviously, you can’t meet with every single person with whom you need to exchange public keys.

Two basic models for key exchange have been developed. The first is a hierarchical approach. At the top is a person or organization that everyone has to trust. This person/organization hands out authority to other organizations or people who can authenticate certain groups of people. These second-tier authenticators then publicly publish the public keys of the people they authenticate. This is the simplest form of this scheme. It’s possible (and probably necessary) to have many more levels than this.

Organizations or individuals can issue a special document known as a “Digital Certificate” saying that they have authenticated a certain person. Then that person can show the digital certificate to others as proof of her identity on the Internet. These digital certificates are usually in a format known as X.509. One company issuing digital certificates is VeriSign (http://www.verisign.com/). To use the Netscape FastTrack Server, you have to acquire a digital certificate from VeriSign, though it should be possible to use other companies in the near future (probably by the time you are reading this).

The other model, known as the Web of Trust, doesn’t depend on being able to trust these higher-level people and organizations. In the Web of Trust, you initially exchange keys with at least a few people you meet in person. Both of you can digitally sign each other’s keys with your names. Then, when others later get your key over the network, they see that it’s signed by the person with whom you exchanged keys in person. If they have exchanged keys in person with that person and trust that person, they then know that they can trust your key.

To make this system work well, you have to decide how much you trust other people’s ability to validate and exchange keys correctly. If you exchange keys with someone you don’t think validates people correctly, you can later ignore any key you get over the network that is signed by that person and not signed by someone you do trust. If you trust their validation techniques, you take any keys signed by them. You can also partially trust them with some implementations of this model. When you start thinking about scaling this model, your head will probably start to spin, but it’s a powerful way for small groups of people to communicate without a lot of overhead.

Most people tend to be rather religious about which model of key exchange is the best. However, in reality, it has been proven mathematically that either model can emulate the other model with a little work. In fact, converting the Web of Trust into the hierarchical approach is almost trivial. An organization only needs to create a key and then sign the keys of people whom they can authenticate. If people know they can trust that organization, they will accept keys that are signed by the organization as being valid.

My personal belief is that both methods are flawed in some ways. Therefore, this ability to twist one model into acting like the other is essential. Sometimes you want to get keys from a central authenticated database, especially when you can’t meet this person in person or can’t meet someone who can meet them to handle the key exchange. At other times, you want to get a key directly from someone (or slightly indirectly, from a source you personally trust), so that you know it’s absolutely correct.

Popular Public Key Packages

The most popular piece of public key encryption software on the Internet is PGP (Pretty Good Privacy). PGP is available for numerous platforms and uses the Web of Trust model of key exchange. To get around the legal restrictions, there’s a U.S. version of the program and a separate international version that was written outside the U.S. Therefore, you can use PGP without worrying about the export controls. (Just don’t carry a copy on your laptop from inside the U.S. to other countries!) In reality, at least two U.S. versions of PGP exist. A version for noncommercial use is freely available by signing some documents with MIT. There’s also a commercial version known as ViaCrypt PGP.

PEM (Privacy-Enhanced Mail) is another algorithm for public key encryption. PEM is based on the hierarchical key exchange model. RIPEM (Riodran’s Internet Privacy-Enhanced Mail) is the reference implementation, which is available from RSA Data Systems. PEM seemed destined for greatness a couple of years ago, but it really has taken a back seat to PGP in actual use.

MOSS (MIME Object Security Services) is intended to correct a couple of the flaws of PEM, one being that PEM’s rigid hierarchies are too strict on many occasions. MOSS relaxes the restrictions somewhat. MOSS is designed to handle MIME messages, unlike PEM. MOSS has too many options, and it might be possible for two different vendors to write MOSS implementations that can’t speak to each other. Some people at the Internet Engineering Task Force (IETF) told me and others to forget PEM, and that MOSS is the algorithm for the future. However, upon researching for this chapter, I found material on the Web indicating that MOSS is a niche system and that PEM was still alive and well. I recommend watching market trends on this debate to see which side is better to use and is gaining market share.

SSL

SSL (Secure Socket Layer) is a security protocol developed by Netscape. In the protocol stack, it runs above the TCP protocol but below the application layer protocols such as NNTP, HTTP, and FTP. While I am writing this, SSL has been implemented only in uses related to HTTP.

SSL enables the client to authenticate the server. It also enables data being transmitted over the Internet to be encrypted. If you are using Netscape Navigator, you can know you are talking to an authenticated server when the key in the lower-left corner of the window is in one piece and not broken. If you have the exportable version of Netscape Navigator, it uses only the 40-bit key discussed earlier (in the section dealing with U.S. export restrictions). So, this isn’t a very secure system.

The current version that is implemented is SSL 2.0. However, the SSL 3.0 specification is available; 3.0 enables the client, as well as the server, to be authenticated.

An SSL server runs on two ports. First, it runs normal, unencrypted as always. Also, it answers on a second port, 443 by default, for encrypted transactions. If your URL for unencrypted transactions ishttp://www.utdallas.edu/, for example, your URL for encrypted transactions is https://www.utdallas.edu/. The only difference is at the beginning of the address: https instead of http.

At the current time, SSL is implemented in the Netscape FastTrack Server, recent versions of the NCSA server, and Open Market’s Secure Web Server. Patches are also available for the popular, free Apache Web server. SSL also is implemented in Netscape Navigator on the client side. For more information, see the following address:

S-HTTP

S-HTTP (Secure HyperText Transfer Protocol) is a higher-level encryption scheme than SSL to protect Web transmission. Although S-HTTP and SSL seem to be in competition, it has been discussed that there’s no reason not to use both in conjunction with each other. In fact, Open Market has implemented both in its Secure Web Server product. Netscape is considering support of S-HTTP as well as SSL in its products. URLs using S-HTTP start with s-http://.

The S-HTTP specification is being developed by CommerceNet and can be seen at the following addresses:

Shen

Shen is a proposal similar in nature to S-HTTP. It hasn’t received widespread support. However, it’s being developed by Phillip Hallam-Baker of the W3 Consortium; and because it is one of the key players in the Web standards world, you should keep an eye on it just in case. The message format it uses is inspired by PEM but unfortunately is not compatible with it. Shen is discussed at these addresses:

S/MIME

S/MIME (Secure/Multipart Internet Mail Extensions) is a standard to exchange e-mail in encrypted form. The specification can be found at http://www.rsa.com/rsa/S-MIME.

Like PGP, the public key encryption is just to manage key exchange; the bulk of the encryption is done with private key encryption algorithms. S/MIME is flexible and enables the use of DES, Triple-DES, and RC2 as private key encryption algorithms.

GSS-API

GSS-API (Generic Security Service-Applications Programming Interface) is a program interface for security that includes both client and server authentication as well as data encryption. It’s “generic” because it was designed to work with any Internet service that needs security. When used with HTTP, the URLs start with gss-http://. GSS-API has its supporters, but hasn’t been deployed much. It’s an interesting approach, though. More information can be found at this address:

SET

SET (Secure Encryption Technology) is a standard for exchanging credit card transactions across the Internet. It was developed by a group including MasterCard, Visa, Netscape, IBM, Microsoft, VeriSign, and GTE. American Express has shown support for it more recently. The major credit card companies state that they don’t see encryption technology as a point of difference between them. They all agree that all transactions should be secure-whether by them or their competition.

Because SET has the backing of so many major players in the electronic commerce world, it definitely bears watching. Expect that you will see the beginning of deployment in late 1996. It also is using X.509 certificates, just like SSL.